Conventional hidden hinge MEMS mirrors, such as those disclosed in U.S. Pat. No. 5,212,582 issued May 18, 1993 in the name of William Nelson, and U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 in the name of Victor Buzzetta et al, include a mirror mounted on the end of a pedestal, extending from a substrate, which are rotatable about a single axis and actuated by electrodes patterned on the substrate below each side of the mirror. In an effort to provide biaxial rotation, Nasiri et al, disclose a MEMS mirror with a complicated hidden lever system, in U.S. Pat. No. 6,533,947 issued Mar. 18, 2003. Unfortunately, the device disclosed in Nasiri et al requires four independent levers and four sets of electrodes equally spaced 90° from each other beneath the mirror, thereby requiring a mirror with a large surface area. Furthermore, an array of such mirrors could not be tightly packed together for reflecting individual wavelengths of light, which has been dispersed in an optical switch. Jung et al disclose a somewhat higher fill factor micro-mirror in an article entitled: “High Fill-Factor Two-Axis Gimbaled Tip-Tilt-Piston Micromirror Array Actuated by Self-Aligned Vertical Electrostatic Combdrives” in the Journal of Microelectromechanical Systems, Vol. 15, No. 3, pages 563 to 571, June 2006; however, the actuation thereof requires eight sets of electrodes spaced apart in a square configuration, thereby increasing the required size of each micro-mirror. Moreover, the comb fingers for the tilt electrode extend perpendicular to the tilt axis, and require relatively fine finger spacing, e.g. 3 um. Piano-MEMS micro-mirrors, which tilt about two perpendicular axes and can be tightly packed together, are disclosed in U.S. Pat. No. 6,934,439 issued Aug. 23, 2005 in the name of the present Applicant. A hidden hinge version of the piano-MEMS micro-mirrors is disclosed in U.S. Patent Publication 2007/0236775 published Oct. 11, 2007 in the name of the present Applicant. The aforementioned piano-MEMS devices pivot about a single centrally located post with the use of torsional hinges and a gimbal ring. Since these devices attract the lower surface of the mirrored platform toward the hot electrodes on the substrate, the precision and maximum tilt angle is limited by the size of the mirror.
Future MEMS mirror arrays for wavelength selective switching call for relatively long and stiff (thick) mirrors capable of tilting in two axes, and a relatively high tilt angle. Hidden hinge designs, in which the mirror is fabricated in a layer above the hinge plane, are attractive to reduce mirror mass moment of inertia and chip size, as the mirror need only be about the same size as the optically active area required.
One solution is disclosed in U.S. Pat. No. 7,952,778 issued May 31, 2011 to Moidu et al illustrated in FIGS. 1 to 4. With reference to FIG. 1 the hidden hinge device includes a substrate wafer 1 formed with a valley section 2 and raised supports 5a and 5b at opposite ends thereof. The substrate wafer 1 is patterned with a rectangular-shaped Y (or tilt) electro-static electrode 3, generally along and symmetrical about the longitudinal X-axis of the device and on one side of the lateral Y-axis of the device, and a C-shaped X (or roll) electro-static electrode 4 extending along one side of the valley section 2, i.e. on one side of the longitudinal X axis and on both sides of the lateral Y axis, symmetrical thereto. The roll electrode 4 includes two large sections, one on opposite sides of the Y-axis, and a thin trace section extending therebetween, providing an area for the tilt electrode 3 to be located between the large sections of the roll electrode 4. Each of the large sections is arranged between the tilt electrode 3 and one of the raised end supports 5a or 5b, and connected through the middle by the elongated trace section, which is thinner than the large sections, and which extends beneath the side of the mirror corresponding to the roll electrode 4. Positioning the roll and tilt electrodes 3 and 4, respectively, along the X axis enables relatively long and thin mirror structures to be positioned relatively close together with only a small air gap therebetween.
With reference to FIG. 2, a ground electrode/hinge wafer 6, processed independent of the substrate wafer 1, is bonded at each end thereof onto the supports 5a and 5b of the substrate wafer 1, suspending the remaining electrode/hinge structure above the valley section 2 of the substrate wafer 1. The electrode/hinge wafer 6 includes an inner, tilting, rectangular ground element or platform 7 pivotable about the lateral Y axis defined by laterally extending torsional tilt (piano) hinge 8. The outer ends of the tilt hinge 8 are fixed to cross braces 9 proximate the lateral Y axis. The tilt electrode 3 is disposed below one side of the tilting ground platform 7, i.e. on one side of the Y-axis for attracting the bottom of one side of the tilting ground platform 7.
The cross braces 9 connect outer rolling structures 11a and 11b, e.g. frames or platforms, forming a rolling ground electrode element 10 with ground electrode surfaces for the hot roll electrode 4, which is disposed below one side of both of the rolling structures 11a and 11b, and below one of the cross braces 9. As a result, the rolling ground electrode element 10 surrounds the tilting ground platform 7. A generally longitudinally extending torsional roll hinge 12 extends from the outer end of each of the rolling structures 11a and 11b to mounting platforms 13, which are mounted on the raised sections 5a and 5b of the substrate 1. The tilting ground element 7 is capable of tilting independently from the rolling ground electrode 10 about the Y-axis, because the tilt hinges 8 extend from the rolling ground electrode 10 and do no resist rotation about the Y-axis. The entire rolling ground electrode 10 along with the tilting ground platform 7 tilt (roll) together about the X-axis via roll hinges 12, because the tilt hinges 8 resist rotation of the tilting element 7 about the X-axis relative to the rolling ground electrode 10. The roll hinges 12 also acts as an electrical connection between ground and external bond pads.
With reference to FIGS. 3 and 4, a mirror wafer 15 is patterned separately from the ground electrode/hinge wafer 6 and the substrate wafer 1 with an upper mirrored platform 16 and pedestal 17 extending downwardly therefrom, which in turn is bonded onto the tilting ground platform 7. The mirror wafer 15 may have stiffening features, such as ribs or bulkheads, extending between the pedestal 17 and the mirror platform 16, if required. Ideally, a plurality of the MEMS mirror platforms 16 are positioned adjacent each other with only a small air gap therebetween for redirecting individual sub-beams from a dispersed beam of light, as disclosed in U.S. Pat. No. 6,934,439 issued Aug. 23, 2005 in the name of Mala et al of JDS Uniphase Inc, which is incorporated herein by reference.
When a potential is applied to the tilt electrode 3 relative to ground, the electrostatic force of attraction between one side of the tilting ground platform 7 and the tilt electrode 3 causes the tilting ground platform 7 and the mirror wafer 15 to tilt, relative to the rolling ground electrode 10, about the Y-axis via tilt hinge 8. Similarly when a potential is applied to roll electrode 4 relative to ground, the electrostatic force of attraction between one side of the rolling ground electrode 10 and the roll electrode 4 causes the entire suspended portion of the ground layer 6, including the rolling ground electrode 10 and the tilting ground platform 7 along with the mirror wafer 15, to tilt about the outer roll hinges 12, i.e. the X axis. The angular position of the tilting ground platform 7 and accordingly the mirror platform 16 can be adjusted according to the amount of voltage applied to the tilt electrode 3 for redirecting a sub-beam of light incident on the mirror platform 16 to any one of a plurality of output ports, as is well known in the art of optical switching. To prevent the sub-beam from momentarily being transmitted to an interim output port physically in between the original output port and the new output port, the roll electrode 4 is activated to rotate the mirror platform 16 out of alignment with any of the output ports until the tilt electrode 3 is activated to tilt the mirror platform 16 to the correct angle corresponding with the desired output port. Then the roll electrode 4 is deactivated bringing the rolling ground electrode 10 back into the rest position with the tilting ground electrode 7 tilted at the correct angle corresponding to the desired output port. Suitable electrode configurations are disclosed in U.S. Pat. No. 6,968,101 issued Nov. 22, 2005, and U.S. Pat. No. 7,010,188 issued Mar. 7, 2006 both in the name of Miller et al to JDS Uniphase Inc, which are incorporated herein by reference.
Unfortunately, the aforementioned device disclosed in U.S. Pat. No. 7,952,778 requires that all of the electrodes are positioned directly beneath the mirror; however, as design requirements call for mirror arrays with tighter pitches the width, i.e. the area, of the roll electrode becomes too small to generate enough electrostatic torque to tilt the mirror as required. For example: a mirror array design with a minimum pitch of 93 um only provides 36 um for the roll electrode, which is 9 um short of the 45 um required to provide the sufficient amount of electrostatic torque.
An object of the present invention is to overcome the shortcomings of the prior art by providing a biaxially pivoting MEMS micro-mirror array device in which the pitch is relatively small, but the tilt electrodes are large enough to sufficiently rotate the mirrors out of alignment with interim ports.